1. Field of the Invention
This invention relates generally to an ultra-broadband ultraviolet (UV) catadioptric imaging microscope system, and more specifically to an imaging system that comprises a UV catadioptric objective lens group and a wide-range zooming tube lens group.
2. Description of the Related Art
Catadioptric imaging systems for the deep ultraviolet spectral region (about 0.19 to 0.30 micron wavelength) are known. U.S. Pat. No. 5,031,976 to Shafer and U.S. Pat. No. 5,488,229 to Elliott and Shafer disclose two such systems. These systems employ the Schupmann achromatic lens principle and the Offner-type field lens. Axial color and primary lateral color are corrected, but not higher order lateral color. This is the limiting aberration in these systems when a broad spectral range is covered.
The above-noted '976 Shafer patent discloses an optical system based on the Schupmann achromatic lens principle which produces an achromatic virtual image. A reflective relay then creates an achromatic real image from this virtual image. The system, reproduced here as FIG. 1, includes an aberration corrector group of lenses 101 for correcting image aberrations and chromatic variation of image aberrations, a focusing lens 103 for receiving light from the group 101 and producing an intermediate image at plane 105, a field lens 107 of the same material as the other lenses placed at the intermediate image plane 105, a thick lens 109 with a plane mirror back coating 111 whose power and position are selected to correct the primary longitudinal color of the system in conjunction with the focusing lens 103, and a spherical mirror 113 located between the intermediate image plane and the thick lens 109 for producing a final image 115. Most of the focusing power of the system is due to the spherical mirror 113 which has a small central hole near the intermediate image plane 105 to allow light form the intermediate image plane 105 to pass through to the thick lens 109. The mirror coating 111 on the back of the thick lens 109 also has a small central hole 119 to allow light focused by the spherical mirror 113 to pass through to the final image 115. While primary longitudinal (axial) color is corrected by the thick lens 109, the Offner-type field lens 107 placed at the intermediate image 105 has a positive power to correct secondary longitudinal color. Placing the field lens slightly to one side of the intermediate image 105 corrects tertiary longitudinal color. Placing the field lens slightly to one side of the intermediate image 105 corrects. tertiary longitudinal color. Thus, axial chromatic aberrations are completely corrected over a broad spectral range. The system also incidentally corrects for narrow band lateral color, but fails to provide complete corrections of residual (secondary and higher order) lateral color over a broad UV spectrum. The above-noted '229 patent to Elliott and Shafer provides a modified version of the optical system of the '976 patent, which has been optimized for use in 0.193 micron wavelength high power excimer laser applications such as ablation of a surface 121′ as seen in FIG. 2. This prior art system has an aberration corrector group 101′, focusing lens 103′, intermediate focus 105′, field lens 107′, thick lens 109′, mirror surfaces 111′ and 113′ with small central opening 117′ and 119′ therein and a final focus 115′ as in the prior '976 patent, but repositions the field lens 107′ so that the intermediate image or focus 105′ lies outside of the field lens 107′ to avoid thermal damage from the high power densities produced by focusing the excimer laser light. Further, both mirror surfaces 111′ and 113′ are formed on lens elements 108′ and 109′. The combination of all light passing through both lens elements 108′ and 109′ provides the same primary longitudinal color correction of the single thick lens 109 in FIG. 1, but with a reduction in total glass thickness. Since even fused silica begins to have absorption problems at the very short 0.193 micron wavelength, the thickness reductions is advantageous at this wavelength for high power levels. Though the excimer laser source used for this optical system has a relatively narrow spectral line width, the dispersion of silica near the 0.193 micron wavelength is great enough that some color correction is still needed. Both prior art systems have a numerical aperture of about 0.6.
Longitudinal chromatic aberration (axial color) is an axial shift in the focus position with wavelength. The prior art system seen in FIG. 1 completely corrects for primary, secondary and tertiary axial color over a broad wavelength band in the near and deep ultraviolet (0.2 micron to 0.4 micron region). Lateral color is a change in magnification or image size with wavelength, and is not related to axial color. The prior art system of FIG. 1 completely corrects for primary lateral color, but not for residual lateral color. This is the limiting aberration in the system when a broad spectral range is covered.
U.S. patent application Ser. No. 08/681,528, filed Jul. 22, 1996, now U.S. Pat. No. 5,717,518, is for a catadioptric UV imaging system with performance improved over the systems of the above-describe patents. This system employs an achromatized field lens group to correct for secondary and higher order lateral color, which permits designing a high NA, large field, ultra-broadband UV imaging system.
Zooming systems in the visible wavelengths are well-known. They either do not require very high levels of correction of higher-order color effects over a broad spectral region, or do require correction, but accomplish this by using three or more glass types. In the deep UV, there are very few materials that can be used for chromatic aberration correction, making the design of high performance, broadband optics difficult. It is even more difficult to correct for chromatic aberrations for ultra-broadband optics with wide-range zoom.
There remains, therefore, a need for an ultra-broadband UV microscope imaging system with wide-range zoom capability.